[mercury-users] Extending a structure data type in Mercury

[Gregory D. Weber]

(Thanks for your response; I've been away; it's nice to be getting back to Mercury.)

I did consider using typeclasses.  If I understand correctly,
typeclasses are like interfaces in Java, so I would have to
declare a type class with some methods, then define
instances ("classes"?) which implement those methods.
I can't have one typeclass be a subclass of another
and inherit methods from it, can I?

So if I were to do this with type classes, I would need to think
of prules, etc., as abstract data types; the type class
would declare methods like prule_key, prule_class, prule_ftable, prule_conds,
and the instances cn2rule and icnrule would each have to define implementations
of those methods.  Correct?

Greg

Ralph Becket <rafe at cs.mu.OZ.AU> writes:

> Gregory D. Weber, Saturday,  4 June 2005:
> > I want to "extend" a structure type by adding fields, like this:
> > 
> > :- type prule ---> prule(key, class, ftable, conds).
> > :- type cn2rule == prule.
> > :- type icnrule ---> icnrule(key, class, ftable, conds, info).
> > 
> > But want icnrule to be considered a subtype of prule,
> > also as a subtype of cn2rule, so that procedures developed for prules 
> > and cn2rules will also work for icnrules.
> > 
> > I've found a solution, but I'm wondering if there's a better way.
> 
> Hmm.  I think you might find it easiest to do this sort of thing using
> type classes.  Would that work?
> 
> -- Ralph

-- 
Gregory D. Weber             http://mypage.iu.edu/~gdweber/
                             Telephone (765) 973-8420
Associate Professor of Computer Science; Chair, Informatics Committee
Indiana University East
2325 Chester Boulevard, Richmond, Indiana 47374-1289, U.S.A.
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